Physiology and Molecular Biology of Stress ... - KHAM PHA MOI

Salt Stress
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important role in signal transduction, as was reported for Arabidopsis AtGSK1, whose
overexpression in transgenic plants was seen in response to salinity conditions (Piao et
al., 2001).
Exogenous ABA can activate transcription of group of genes induced by salt
or water stress, while other stress-inducible genes were not activated, suggesting ABAdependent
and ABA-independent signaling pathways (Shinozaki and Yamaguchi-
Shinozaki, 1997). There is information on ABA-dependent transcriptional activation
related to Ca 2+ -dependent protein kinases (CDPKs) (Sheen, 1996), and ABA-independent
transcription factors such as dehydration response element (DRE) binding proteins
(Liu et al., 1998). The ABA-dependent and ABA-independent expression pathways
were also obtained in the case of the LEA gene Dc3 from carrot in transgenic
tobacco (Siddiqui et al., 1998). The expression of osmotically induced gene saltT in rice
is initiated by either salinity or ABA through antagonistic signal transductional pathways
(Garcia et al., 1998). Drought and salt stress induce ABA biosynthesis largely
through transcriptional regulation of ABA biosynthesis genes (Xiong and Zhu, 2003).
8.4. Detoxification and ROS Scavenging
Oxidative stress is an additional phenomenon of stress impact on plants. This secondary
effect emerges as a consequence of hyperosmolarity caused by imposing of plants
to drought or salt stress conditions, resulting in appearance of the reactive oxygen
molecules, such as hydrogen peroxide, hydroxyl radicals and superoxide anions (Xiong
et al., 2002). The scavenging of reactive oxygen species (ROS) is associated with both
activity of the enzymes involved in antioxydative processes of the cell (particularly
superoxide dismutase, glutathione peroxidase and catalase) and the presence of
osmoprotectant compounds, including mannitol and proline (Xiong et al., 2002). Harmful
influence of ROS on cell macromolecules may be also alleviated by the activity of
antioxidant compounds such as ascorbic acid, glutathione, thioredoxin and carotenoids
(Xiong and Zhu, 2002). Preincubation of particular enzymes in vitro with a various
osmolytes prevented a salt-induced inhibitory effects on the enzyme activity (Ghosh et
al., 2001). Participation of antioxidative system in responses to salt stress was studied
in tomato and its wild salt-tolerant relative Lycopersicon pennellii (Shalata and Tal,
1998). Activities of superoxide dismutase (SOD), ascorbate peroxidase (APX) and
dehydroascorbate reductase (DHAR) under concentration of 100 mM NaCl were inherently
higher in L. pennellii than in L. esculentum.
Free radical-mediated damage of membrane might be a component of the cellular
toxicity of salt-stressed rice seedlings, according to the ability of the salt-tolerant
varieties to maintain the specific activity of antioxidant enzymes (SOD and peroxidase),
in difference to salt-sensitive varieties (Dionisiosese and Tobita, 1998). Higher capacity
for ROS scavenging was observed in tomato calli tolerant to 50 mM NaCl in comparison
with control, on the basis of the activity of lipoxygenase and antioxidant enzymes
(Rodriguezrosales et al., 1999). Exposure of wheat genotypes to long-term salt stress